I am pretty sure it was via StumbleUpon. Which seems so appropriate because their little self sales line is that they are "discovery engine that finds the best of the web.". And ironic because once I found Metafilter I was a whole lot less interested in other StumbleUpon suggestions..
Getting Dragon Age from Shothotbot in SQ exchange. Wow!
The first time MetaFilter really clicked for me was the first time I saw a day's worth of posts on the Blue. I felt like I was viewing a giant chewy chocolate brownie with excellent dark chocolate frosting.